1. Field of the Invention
The invention relates generally to forage harvesters and, more particularly to a cutterhead of a forage harvester.
2. Related Technology
Self-propelled forage harvesters are typically equipped with open or closed cutterhead having detachable knifes in order to chop picked-up crop into small pieces, which serve as fodder. Attached on the cutterhead body are knife retainers, which in turn carry the knives.
Conventional knife retainers are disposed on cutterheads parallel or at an angle to the axis of rotation of the cutterhead. Knife retainers arranged at an angle (i.e. incline) to the axis of rotation are used to perform a so-called pull-through cut with the knives attached thereon upon a rotating motion of the cutterhead. In this configuration, the cutting edges of the knives also form an angle with respect to the direction of rotation of the cutterhead that is between 90° and 0° and with respect to the edge of shear bars disposed parallel to the axis of rotation of the cutterhead. The knives, which are flat, have a leading cutting edge in the direction of rotation that extends parallel to the fastening area fixed to the knife retainer and non-parallel with respect to the axis of rotation, as described in DE 88 13 953 U. Alternatively, the knives are bent in a spiral shape as disclosed in U.S. Pat. No. 3,805,660. It has also been suggested to arrange trapezoidal, angled knives on knife retainers; the knife retainers extending parallel to the axis of rotation of the cutterhead in order to achieved a pull-through cut as disclosed in U.S. Pat. No. 5,544,826, which is herein incorporated by reference. A cutterhead having knife retainers extending parallel to the axis of rotation of the cutterhead and cutting edges of the knives is described for example in DE 27 37 684 A. The knife retainers perform a so-called straight or chopping cut.
One disadvantage of a pull-through cut is that a force component develops that pushes the material to be cut out of the opening formed by the cutting edge of the knife and the edge of the shear bar, which act like scissors. The problem of material to be cut pushing out of the cutting elements increases with increasing size of the pulling angle. It also increases with the position of the knife's tapered cutting edge with respect to the direction of rotation or the edge of the shear bar, with further rounding of the cutting edges and the shear bar edges due to wear, and with decreasing coefficients of friction for the material to be cut, particularly if the moisture content of the material to be cut changes. If the material to be cut is automatically transported between the shear bar's edges and the knives' cutting edges that are disposed at an angle to the direction of rotation and retained there, this action counteracts the outward push of the material. If this forced feed and fixation of the material does not exist, then the material to be cut may escape the pushing force by moving sideways. In this configuration, the material is only partially cut or not at all. Consequently, the true mean length of the cut material shift towards a greater mean length of cut in the chopped pile at constant feed speeds, i.e. at lengths of cuts that are set to a constant length.
Despite the above-described problem, the pull-through cut has been widely used over the last few decades since it achieves good cutting results in many crop types, such as corn. In other crop types, such as grass, its cutting results are frequently less satisfactory. Replacing the cutterhead in an embodiment having both knife retainers and cutting edges extending parallel to the axis of rotation can be difficult.